In automotive vehicles, engine room space has necessarily been reduced for keeping up with the propagation of FF (front engine-front wheel driven) cars aiming at reduction in size and weight of the vehicles, and with the demand for increased cabin space. Accordingly, the size and weight of the electrical components and accessories such as alternators, electromagnetic clutches for car air conditioners, idle pulleys, or electric fan motors have further been reduced. At the same time, the electrical components and accessories are demanded to achieve higher performance and higher output power. Therefore, in an alternator, for example, reduction in output power caused by size reduction is compensated by increasing the designed speed. Further, to keep up with the demand for quiet operation, the degree of hermetic sealing of the engine room is advanced and hence the engine room tends to be heated, so that components capable of withstanding higher temperatures are required.
In these electrical components and accessories, rolling bearings are used, which are lubricated mainly with grease. However, there has been reported that, as the rotational speed of the bearings and the load applied to the bearings are increased due to the sever operational conditions, the service life of the rolling bearings tends to be prematurely terminated because of flaking of the race way of the bearings.
In an attempt to prevent the flaking of the race way, Japanese Laid-open Patent Application No. 3-210394 discloses a method of adding a passivating agent to the grease, and Japanese Laid-open Patent Application No. 6-803565 discloses a method of adding an antimony compound or a molybdenum compound to the grease to extend the flaking life of the race way of the bearings.
In the method of adding a passivating agent to the grease mentioned above, however, if a typical passivating agent sodium nitrite is added to a grease containing an amine antioxidant for improving the grease service life at elevated temperatures, sodium nitrite reacts with the amine antioxidant to generate a toxic N-nitrosamine compound as a by-product. Therefore, it is difficult to produce a grease which prolongs the flaking life of the race way of bearings while maintaining its service life at elevated temperatures at high level and its harmful effect on environment at low level. Further, passivating agents other than sodium nitrite, and the antimony compound and the molybdenum compound also have problems in that they contain heavy metals, which have harmful effects on human bodies as well as environment.